Venturing through the mouth the Doctor & Leela find a spaceship and meet it's occupants the Tesh. The Tesh & the Sevateem are descendants of the original crew: The Tesh from the technicians that stayed with the ship and the Sevateem from the survey team that went to survey the planet. Xoanon is the ship's computer repaired long ago by the Doctor but now suffering from a split personality. It has been conducting a eugenics experiment using the Sevateem & Tesh and has now sent the monsters to kill the Sevateem. Using the communications equipment to speak to the high priest Neeva the Doctor summons the Sevateem through the mouth of the stone face. The Doctor goes to confront Xoanon but is mentally assaulted by the computer forced to the floor as it bellows "who am I?"

That's one big information dump trying to explain what's going on. The concepts are good but the execution, especially those appalling Tesh uniforms and make-up, lets it down.

Two of the Xoanon voices are provided by actors from the next story:Rob Edwards and Pamela Salem are both in Robots of Death as Chubb & Toos respectively with Salem returning as Professor Rachel Jensen in Remembrance of the Daleks. A third voice is provided by Tom Baker and the fourth by Anthony Frieze, the winner of a competition to visit the Doctor Who studios.

So when did the Doctor first come to the planet? A question that has exercised many fans' minds over the years. Some suggest a trip soon after his regeneration during Robot. I'm not sure that quite works, I can't see a suitable point for it to have happened. A better scenario would be (see Pyramids of Mars 3) for the Doctor to have dropped Sarah off in 1975 at the end of Planet of Evil, had some adventures including his earlier visit here, and then picked her up again in 1980 thus fixing the UNIT Dating problems in one swoop. A third option involves adventures between Deadly Assassin and here. This idea appeals to me: not only does The Doctor leave Gallifrey alone but he arrives here alone: who knows how long he'd been travelling for by himself? Centuries even. From this point on the Doctor doesn't refer to UNIT and recently seen events, from our point of view, so much so it's plausible that a large amount of time may have passed for him. The show taking a break between these two stories as well makes it all the more tempting to imagine that all sorts of things may have happened between them that we don't see.